Methods for brazing together heat exchangers comprised of a plurality of stamped sheet metal constituent parts can be roughly categorized into flux requiring and fluxless methods. Flux, when needed, is a caustic chemical that melts at just below the temperature at which a layer of braze material clad to the surface of the part melts. The melted flux serves both to dissolve, and prevent the further formation of, metal oxide layers on the metal part outer surfaces. The oxides would otherwise prevent the rough wetting and flow of melted braze material into the joint interfaces, without which solid braze seams cannot form. A separate chemical flux application is not needed when either the process used, or the metal material used, or both, do not require it. For example, in so called vacuum brazing, almost all atmospheric gases, including harmful oxygen, are removed from the braze oven, preventing the formation of oxides at braze temperature. Vacuum brazing obviously requires special equipment, and evacuation, by definition, cannot be part of a continuous, flow through brazing process. Moreover, vacuum brazing is really only useful if the metal itself is not one that is particularly prone to oxidation, such as stainless steel, or an alloy of aluminum containing a sufficient percentage of magnesium to inherently scavenge existing oxide layers without the need for chemical flux. Such alloys are not always desirable.
With flow through braze processes, evacuation is not feasible, but an inert atmosphere can be provided continually around the part to exclude most oxygen. Most often, the part is preheated to just below the braze melt temperature in an initial prechamber, in which the atmosphere need not be as strictly controlled, before entering the braze oven proper. Even so, not all oxygen can be excluded, so that a flux layer is needed, both to prevent further oxide formation and to dissolve the existing oxide layers, especially with common aluminum alloys that do not contain magnesium. Two common flux layer application methods are aqueous fluxing, in which a water slurry of flux is applied to the part, which must be oven dried separately before going to braze. More recently, dry surface flux application by electrostatic methods has been used, similar to dry paint powder coating methods. It has also been proposed in U.S. Pat. No. 5,174,490 issued Dec. 29, 1992 to Koisuka et al. to assist the electrostatic flux coating process by water spraying the part first, so that the dry flux powder will stick to the wet surface, in addition to the electrostatic force. The braze oven preheat process is sufficient to dry the lightly wetted part, so that a separate flux dry oven is not needed, as with a flux slurry bath.
Fluxing and brazing must also take into account the fact that formed metal parts, especially stamped aluminum alloy parts, are invariably stamped with a suitable lubricating oil or grease, a film of which adheres to the metal surfaces after stamping. Several processes exist for removing the forming oil residue, including solvent baths and vapors, or aqueous wash, which is environmentally preferable. With fluxless brazing processes, it has been recognized that so called thermal degreasing of the surfaces is possible instead of solvent or water wash. In thermal degreasing processes, the oil covered part is heated sufficiently to remove the surface oil, either by volatilization or, reduction with a reactive atmosphere. For example, in U.S. Pat. No. 3,747,199 issued Jul. 24, 1973 to Swaney, Jr., it is recognized that with a magnesium containing aluminum alloy, which is also vacuum brazed, the evacuated preheat chamber alone will successfully clean the part oil film, since the heat and vacuum cooperate to both evaporate the oil film and transport its vapor out of the prechamber. U.S. Pat. No. 5,016,809 issued May 21, 1991 to Winterbottom et al. teaches a separate thermal degreasing step, not in the preheat chamber itself, but in a special reactive atmosphere of oxygen, ammonia or hydrogen, and at a temperature low enough to actually preserve a metal oxide layer containing the necessary magnesium. Again, a flux layer is not used if needed.
In the case of stainless steel parts, which are vacuum brazed without flux, it is difficult to pre-clad the entire surface with a layer of braze material, as with aluminum alloy sheet. Instead, an adhesive tape of braze material is generally pre-applied directly to the joints when the constituent metal parts are fixtured, prior to brazing. As an alternate to the adhesive braze tape, a very complex process has been proposed to actually use some of the residual forming oil film as part of an effectively adherent layer at the joints to retain braze powder. In U.S. Pat. No. 5,431,330 issued Jul. 11, 1995 to Wieres, the oil covered part is heated at a precisely control temperature range to remove only its more volatile constituents, while leaving the heavier, and presumably more viscous, components, as a residue. Then, as the part cools to a defined residual temperature well above room temperature, the part with viscous oil residue is swabbed with a dilute aqueous surfactant solution of citric acid and detergent. Following this, the part is reheated to another controlled temperature range and dried in an air flow. This precisely controlled heating, swabbing, reheating and air drying process is claimed to form a composite sticky film concentrated at the joints. Then, the part is physically dipped into a supply of nickel based brazing powder with a controlled grain size, some of which adheres to the sticky layers. A separate adhesive layer can be sprayed over the adhering powder layer, as well. Finally, the part with its adherent nickel powder layer is brazed which, for stainless, would be a vacuum brazing process again, without the need for flux. There is no teaching in the patent about flux coating, of course, since the process is fluxless. There is also no suggestion that the part with its sticky composite layer of oil residue and soap surfactant could be electrostatically coated with brazing material powders, nor would such a suggestion be expected, because of what the prior art teaches about electrostatic powder coating in general, described in more detail below
In the past, flux coating of aluminum alloy sheet stamped parts has been by processes other than electrostatic powder coating. The clear teaching of the prior art is that all forming oil film residue must be removed before the flux is applied. For example, in U.S. Pat. No. 4,923,110 issued May 8, 1990 to Carmichael et al., which claims a process for using protective flux coatings, it is recited that "The mating surfaces of the components . . . are initially cleaned by known cleaning process, which may include chemical and/or mechanical cleaning, to remove all dirt, grease, oil, oxides, moisture, and other oxides therefrom". This is inherently obvious for all water based or water assisted flux application processes, since oil and water are proverbially incompatible. But even in U.S. Pat. No. 3,667,111 issued Jun. 6, 1972 to Chartet, which claims a process for application of an anhydrous (dry) flux at high heat, it is recited that the parts "must obviously be free of all direct, particularly traces of grease, which would risk spoiling the quality of the brazing joints." Interestingly enough, the teaching of U.S. Pat. No. 3,747,199, that in fluxless brazing, the degreasing step could be delayed until the preheat stage, seems to have been less than universally recognized, and the patent has not, as of this writing, even been cited in any later patent. The later U.S. Pat. No. 3,937,387 issued Feb. 10, 1976 to Fletcher et al., which claims a method of fluxes brazing, says that "In preparing the aluminum containing surfaces or parts for fluxes brazing and or diffusion bonding, the surfaces to be welded must first be thoroughly cleaned to remove all grease and the aluminum oxide coating."
A newer dry flux coating process of recent interest is electrostatic powder coating. Electrostatic coating for paint and clear coat powders has been widely used for perhaps twenty years, and there is a great deal of existing literature describing how it should be done. In general, a cleaned metal part capable of being electrically grounded is exposed to a cloud of electrically charged particles, either sprayed from a gun or suspended in a fluidized bed. The process has been slower to gain acceptance in applying powdered fluxes, because their heavier and smaller grain size makes them more difficult to spray or fluidize than paint particles. Also, the economic incentive to switch from conventional wet slurry and drying oven systems to dry electrostatic powder fluxing systems is not that high, since the only equipment and step eliminated thereby is the wet flux dry oven. With either system, the parts, according to conventional teaching, must still be thoroughly and rigorously cleaned first, by whatever cleaning process, to remove all oil residue. The conventional wisdom is that the oil cleaning step cannot possibly be delayed until the braze preheat chamber, as with fluxless vacuum brazing, since the part must be fluxed first, and cleaned of oil first.
Again, if there is one recurrent theme with electrostatic dry coating of any powder, it is the absolute necessity for initial part surface cleaning of all contaminants, and especially of oil and grease film. As to electrostatic flux powder coating specifically, an abstract from the February 1996 issue of Aluminum Today, describes the use of an electric thermal degreasing oven to render the surface suitable for flux coating. A dry oil cleaning step is simply substituted for aqueous wash. As to electrostatic application of any dry powder in general, the teaching is just as clear. In the October 1997 issue of Products Finishing magazine, a powder coating clinic teaches regarding clear coating aluminum that "The key word here is `bare.` You must have a surface that is completely free of both organic (dirt and oils) and inorganic (oxides) soils." A Society of Manufacturing Engineers training course on the "Fundamentals of Powder Coating" teaches that "A system for the application of powder is comprised of a pretreatment process (usually a spray washer) . . . ." Another SME publication, "User's Guide to Powder Coating," states that "Considering all key elements involved in a high quality powder-coated product, one truly stands alone--cleaning" (emphasis as in original). This last statement says it best. The one universally perceived truism for electrostatic dry powder coating is that the surface must be absolutely clean first, and especially clean of oil residue, and no one skilled in the art would think to violate that near taboo.